Financing - Activities, PNM and TNMP (Details)								1 Months Ended	12 Months Ended
	Feb. 01, 2018 USD ($)	Jul. 28, 2017 USD ($)	Feb. 01, 2016 USD ($)	Dec. 21, 2015 USD ($)	Aug. 11, 2015 USD ($)	May 08, 2015 USD ($)	Jan. 01, 2015 USD ($)	Jun. 21, 2016 USD ($)	Dec. 31, 2017 USD ($) derivative	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Feb. 20, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jul. 20, 2017 USD ($)	Jun. 14, 2017 USD ($)	Dec. 21, 2016 USD ($) loan	Oct. 21, 2016 USD ($)	Sep. 27, 2016 USD ($)	May 20, 2016 USD ($)	Dec. 17, 2015 USD ($)	Sep. 30, 2015	Jun. 01, 2015 USD ($)	Mar. 09, 2015 USD ($)	Jan. 01, 2014 USD ($)
Debt Instrument [Line Items]
Outstanding amount									$ 305,400,000	$ 287,100,000
Amount outstanding										100,000,000	$ 50,000,000
Payment of debt									274,070,000	303,793,000	333,066,000
Fixed interest rate																					1.927%
PNMR 2016 One-Year Term Loan and PNMR 2016 Two-Year Term Loan
Debt Instrument [Line Items]
Number of loan agreements (in loans) | loan																2
PNMR 2016 One Year Term Loan
Debt Instrument [Line Items]
Outstanding amount									$ 100,000,000	100,000,000						$ 100,000,000.0
Variable interest rate									2.32%
PNMR 2016 Two-Year Term Loan due December 2018
Debt Instrument [Line Items]
Loan entered into																100,000,000.0
Variable interest rate									2.32%
Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Aggregate principal amount									$ 50,000,000.0
Time hedged in interest rate cash flow hedge									4 years
Level 2 | Cash Flow Hedging
Debt Instrument [Line Items]
Fair value gain (loss, less than)									$ 200,000	(100,000)
Notes Payable to Banks | PNMR Term Loan Agreement
Debt Instrument [Line Items]
Outstanding amount																								$ 100,000,000.0
Amount outstanding				$ 150,000,000.0
NM Capital | San Juan Generating Station | Coal Supply
Debt Instrument [Line Items]
Payments to fund long-term loans to unaffiliated third party			$ 125,000,000
PNMR | PNMR 2016 Two-Year Term Loan due December 2018
Debt Instrument [Line Items]
Loan entered into									100,000,000	100,000,000
Public Service Company of New Mexico
Debt Instrument [Line Items]
Outstanding amount									39,800,000	61,000,000
Payment of debt									$ 232,000,000	271,000,000	$ 214,300,000
Public Service Company of New Mexico | NMPRC
Debt Instrument [Line Items]
Requirement to obtain approval for any financing transaction, period of time (more than)									18 months
Line of Credit
Debt Instrument [Line Items]
Debt-to-capital ratios (less than or equal to)									65.00%
Line of Credit | Texas-New Mexico Power Company
Debt Instrument [Line Items]
Debt-to-capital ratios (less than or equal to)									65.00%
PNMR 2015 Term Loan Agreement
Debt Instrument [Line Items]
Loan entered into																$ 150,000,000.0							$ 150,000,000.0
Stated percentage									2.34%
Unsecured Debt | Senior unsecured notes, 9.25% due 2015
Debt Instrument [Line Items]
Aggregate outstanding principal amount																								$ 118,800,000
Stated percentage																								9.25%
Unsecured Debt | Public Service Company of New Mexico
Debt Instrument [Line Items]
Shelf registration statement, capacity of issuance (up to)									$ 475,000,000
Unsecured Debt | Public Service Company of New Mexico | 3.85% due August 2025
Debt Instrument [Line Items]
Aggregate outstanding principal amount					$ 250,000,000.0				250,000,000	250,000,000
Stated percentage					3.85%
Unsecured Debt | Public Service Company of New Mexico | PNM 2017 Term Loan Agreement
Debt Instrument [Line Items]
Loan entered into									200,000,000	0
Unsecured Debt | Public Service Company of New Mexico | 7.95% due May 2018
Debt Instrument [Line Items]
Aggregate outstanding principal amount									$ 350,000,000	350,000,000			$ 450,000,000.0
Stated percentage									7.95%
Extinguishment of debt		$ 350,000,000
Unsecured Debt | Public Service Company of New Mexico | 7.50% due August 2018
Debt Instrument [Line Items]
Aggregate outstanding principal amount									$ 100,025,000	100,025,000
Stated percentage									7.50%
BTMU Term Loan Agreement | NM Capital
Debt Instrument [Line Items]
Loan entered into			$ 125,000,000.0						$ 50,100,000
Stated percentage									4.13%
Estimated principal payments									$ 10,600,000
BTMU Term Loan Agreement | NM Capital | Subsequent Event
Debt Instrument [Line Items]
Loan entered into												$ 45,100,000
Payment of debt	$ 5,000,000
Excess amount of scheduled payment	3,300,000
Westmoreland loan | NM Capital | Subsequent Event
Debt Instrument [Line Items]
Payment of debt	5,600,000
Excess amount of scheduled payment	$ 4,800,000
Letter of Credit | PNMR | Letter or credit, 30 mil JP Morgan
Debt Instrument [Line Items]
Aggregate principal amount																	$ 30,300,000.0
PNM 2014 Term Loan Agreement | Public Service Company of New Mexico
Debt Instrument [Line Items]
Repayments of debt					$ 175,000,000.0
PNM 2014 Multi-Draw Term Loan Agreement | Public Service Company of New Mexico
Debt Instrument [Line Items]
Loan entered into							$ 125,000,000
Outstanding borrowings						$ 25,000,000	$ 100,000,000
Repayments of debt								$ 125,000,000.0
Unsecured notes, Pollution control revenue bonds | Public Service Company of New Mexico | Senior unsecured note, PCRB Due 2043, at 4 percent
Debt Instrument [Line Items]
Aggregate outstanding principal amount																								$ 39,300,000
Unsecured notes, Pollution control revenue bonds | Public Service Company of New Mexico | Senior Unsecured Note PCRB Dude 2020 at 2.45 Percent
Debt Instrument [Line Items]
Aggregate outstanding principal amount																						$ 39,300,000
Stated percentage																						2.45%
PNM 2016 Term Loan Agreement due 2017 | Public Service Company of New Mexico
Debt Instrument [Line Items]
Aggregate principal amount																			$ 175,000,000.0
Senior Unsecured Notes | Public Service Company of New Mexico | Senior Unsecured Note Agreement (SUNs)
Debt Instrument [Line Items]
Aggregate principal amount		$ 450,000,000.0
Debt to capital ratio		65.00%
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 1 Point 875 Percent, due 2021
Debt Instrument [Line Items]
Stated percentage																		1.875%
Aggregate principal amount																		$ 146,000,000.0
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 2.125 Percent, due 2040
Debt Instrument [Line Items]
Stated percentage							2.125%
Aggregate principal amount							$ 37,000,000.0
Pollution Control Bonds | Public Service Company of New Mexico | Pollution Control Revenue Bonds 2.45 Percent, due 2042
Debt Instrument [Line Items]
Stated percentage							2.45%
Aggregate principal amount							$ 20,000,000.0
First Mortgage Bonds | Texas-New Mexico Power Company | 4.03% due 2024, Series 2014A
Debt Instrument [Line Items]
Aggregate outstanding principal amount									$ 80,000,000	$ 80,000,000
Stated percentage									4.03%	4.03%
First Mortgage Bonds | Texas-New Mexico Power Company | 3.53% due February 2026
Debt Instrument [Line Items]
Aggregate outstanding principal amount									$ 60,000,000	$ 60,000,000
Stated percentage									3.53%	3.53%										3.53%
Aggregate principal amount																				$ 60,000,000.0
JPMorgan Chase Bank, N.A. | PNM 2016 Term Loan Agreement due 2017 | Public Service Company of New Mexico
Debt Instrument [Line Items]
Aggregate principal amount										$ 175,000,000.0
JPMorgan Chase Bank, N.A. | Notes Payable to Banks | Public Service Company of New Mexico | PNM 2017 Term Loan Agreement
Debt Instrument [Line Items]
Stated percentage									2.29%
Aggregate principal amount														$ 200,000,000.0
Scenario, plan | Senior Unsecured Notes | Public Service Company of New Mexico | SUNs, Issuance in May 2018
Debt Instrument [Line Items]
Aggregate principal amount		$ 350,000,000.0
Scenario, plan | Senior Unsecured Notes | Public Service Company of New Mexico | SUN's, Issuance in August 2018
Debt Instrument [Line Items]
Aggregate principal amount		$ 100,000,000
Scenario, plan | First Mortgage Bonds | Texas-New Mexico Power Company | Interest Rate of 3.22%, Due 2027
Debt Instrument [Line Items]
Stated percentage															3.22%
Aggregate principal amount															$ 60,000,000.0
Interest rate contract
Debt Instrument [Line Items]
Number of derivative instruments held | derivative									3
Interest rate contract one | Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Fixed interest rate									1.926%
Interest rate contract two | Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Fixed interest rate									1.823%
Interest rate contract three | Variable Rate Short-Term Debt
Debt Instrument [Line Items]
Fixed interest rate									1.629%
Interest rate contract three | Level 2 | Cash Flow Hedging
Debt Instrument [Line Items]
Fair value gain (loss, less than)									$ 1,400,000